1. Field of the Invention
The present invention relates to a rear projection television or rear projection display device and a projecting method thereof and, particularly, to a rear projection display device for enlarging and projecting an image on an image display element onto a projection screen.
2. Description of the Prior Art
Conventional rear projection display devices will be described with reference to FIG. 1 to FIG. 3. FIG. 1 is a cross section of a conventional rear projection television, showing its inner construction. As shown in FIG. 1, a light beam emitted from a projector 101 provided on a rear portion of a bottom plate of a casing of the rear projection television is reflected by a first flat mirror 102 provided in a lower front portion of the casing and, then, a second flat mirror 103 provided on a rear wall of the casing, and, finally, focused on a projection screen 104 provided in a front plate of the casing.
FIG. 2 is a partially cross-sectioned perspective view showing an inner construction of another conventional rear projection display device. As shown in FIG. 2, a light beam emitted from a projector 101 provided on an upper plate of a casing of the conventional device is reflected down by a first flat mirror 102, reflected rearward by a second flat mirror 103 and, then, reflected forward by a third flat mirror 109 onto a projection screen 104 provided in a front plate of the casing. In FIG. 2, thickness, that is, depth size, of the device is reduced by increasing the number of reflections of the light beam projected from the projector 101. A rear projection display device of the above-stated type is disclosed in, for example, JP H4-107521A.
FIG. 3 illustrates a generation of stray light in a conventional rear projection display device such as shown in FIG. 2. In FIG. 3, a light beam, which is emitted from a projector 101 and reflected onto a projection screen 104 by flat mirrors 102 and 103, is partially reflected by a surface of the projection screen 104 at a time when the light beam is incident obliquely on the projection screen. The light portion 111 reflected by the projection screen 104 is reflected by the flat mirrors 103 and incident again on the projection screen 104 as a light beam 112. The light beam 112 appears on the screen as a stray light, causing a ghost image on the screen.
Similarly, an external light beam such as light from a room illumination lamp entered into the casing may generates stray light with which a ghost image may also appear on the screen.
In such conventional rear projection display device as mentioned above, there are many problems.
A first one of them is that a space for receiving one of the flat mirrors must be provided in a portion of the casing of the rear projection television below the screen thereof. Such space may form a skirt portion, which is undesirable in appearance of the rear projection display device and is an obstacle to reduction of the size of the rear projection display device.
A second problem is that the length of the conventional rear projection television in depth becomes large due to long projection light path of the rear projection television. That is, in the conventional projection using projection lenses, the projection path length tends to become large due to the design of the lens. Even if the light path is shortened by folding the light path by using a plurality of reflection mirrors, the size of the rear projection television in depth becomes similar to that of a CRT monitor at best.
A third problem is that stray light, which is a cause of ghost image on the screen, is generated by reflection of a light beam at an incident surface of the screen and/or external light entering into the rear projection television.
A fourth problem is that, when a large number of reflection mirrors are used, the luminance of an image on the screen is degraded due to reflectivity problems of the respective reflection mirrors.
A fifth problem is that, due to the presence of the skirt portion of the conventional rear projection television, it is impossible to manufacture a multi rear projection television including a plurality of rear projection display devices arranged in matrix.
In manufacturing such multi rear projection television, it is important to make junctures between screen portions inconspicuous. Therefore, if the rear projection television has a skirt portion, it is impossible to manufacture the multi rear projection television. In order to solve this problem, it is general in the conventional multi rear projection television to project light beams emitted by projectors onto a screen without folding them by using reflection mirrors. In such case, there are new problems of necessity of large scale construction in setting a multi rear projection television, increase of cost for manufacturing the multi rear projection television, impossibility of mass production of the multi rear projection television and necessity of a room for housing the multi rear projection television, which is large in depth enough to obtain a required projection length without using the folding of light beam.
As means for solving some of these new problems, JP H5-88264A discloses a construction of a rear projection television in which a screen is provided on a whole front surface of a casing thereof, a projector is arranged such that light emitted therefrom is directly directed to an upper plate of the casing and a large size reflection mirror is provided obliquely on a lower surface of the upper plate to reflect the light emitted from the projector onto the screen. With this construction of the rear projection television, there is no skirt portion provided in the casing. However, since the large reflection mirror is provided obliquely on the side of the upper plate such that an incident angle of light to the screen becomes not substantially large, the size of the casing in depth is reduced to only about a half to one third the size, that is, a diagonal size, of the screen at best. Further, since the projector is unnaturally arranged to project light upward, the maintenance of the rear projection television becomes troublesome.
An object of the present invention is to provide a rear projection television having a size, which is reduced by removing a skirt portion thereof and reducing a depth length thereof, and a projection method for use in the rear projection television.
Another object of the present invention is to provide a rear projection television capable of improving the quality of image projected onto a screen thereof by removing stray light projected onto a screen, with which a ghost image is produced on the screen, and a projection method for use in the rear projection television.
Further object of the present invention is to provide a rear projection television capable of improving luminance of an image on a screen by reducing the number of flat reflection mirrors and a projection method for use in the rear projection television.
In a rear projection television according to the present invention, an end side flat reflection mirror in a casing thereof is arranged in the vicinity of an upper edge portion of a projection screen to reflect light beam emitted from a projector to a whole rear surface of the screen. The rear projection television according to the present invention is featured by that an optical axis of the light beam projected up to the end side flat reflection mirror and the screen is slanted to the screen to gradually reduce a distance between the optical axis and the screen.
In such rear projection television, it is a feature that a focusing optical system constructed with a plurality of focusing mirrors for expanding and projecting the light beam is provided between the end side reflection mirror and the projector. The focusing optical system may be constructed with a first mirror for reflecting a light from an image display element for forming an image information, a second mirror for reflecting a light reflected by the first mirror, a third mirror for reflecting light reflected by the second mirror and a fourth mirror for reflecting light reflected by the third mirror. It is preferable to arrange the first to fourth mirrors such that the light reflected by the fourth mirror is incident on the end side flat reflection mirror.
The rear projection television according to the present invention is featured by further comprising a flat reflection mirror provided on a rear wall of the casing such that the light beam from the projector is directed to the end side flat reflection mirror.
In the rear projection television according to the present invention, it is preferable that an angle of a surface of the end side flat reflection mirror with respect to a surface of the screen is in a range from 70 degrees to 120 degrees. Further, it is preferable that an angle of the optical axis of the light beam, which is reflected from a center of the end side flat reflection mirror and incident on the center of the screen, with respect to a center normal line of the screen is 45 degrees or more.
According to the present invention, the depth size of the casing of the rear projection television can be one fifth a diagonal size of the screen or smaller.
The projection screen of the present invention preferably has a laminated structure of a full reflection type Fresnel lens and a lenticular lens and, particularly, an optical axis of the Fresnel lens is positioned outside the screen.
The rear projection television according to the present invention has a final stage mirror for folding the optical path, which is provided on the side of an upper plate or on the bottom side plate of the casing, and the focusing optical system may be constructed with a plurality of lenses. Further, the image display element may be constructed with a liquid crystal panel, which displays an image by reflecting and blocking light from a light source.
Alternatively, the image display element may be constructed with a DMD (Digital Micromirror Device), which is a collection of micro reflection mirrors for displaying an image by reflecting a light beam from a light source in an arbitrary direction.
A projection method of the rear projection television, according to the present invention, is featured by that the focusing optical system for enlarging and projecting an image on the image display element and focusing it on the projection screen and reflection mirrors are combined and arranged such that an optical axis of an incident light to a final one of the reflection mirrors is slanted to the screen to gradually reduce a distance between the optical axis and the screen.
Particularly, the present projection method is featured by that an incident angle of the light on the screen is 45 degrees or more. Further, the present projection method is featured by that a center of the focused image is different in position from the optical axis of the focusing optical system. Further, the projection method is featured by that the optical axis of the light beam reflected by one of the reflection mirrors, which is immediately preceding the final reflection mirror, is slanted to the screen to gradually reduce a distance between the optical axis and the screen.